courseware Scheduling of Distributed Real-Time Systems Jan Madsen Informatics and Mathematical Modelling Technical University of Denmark Richard Petersens Plads, Building 321 DK2800 Lyngby, Denmark

SoC-MOBINET courseware(c) Jan Madsen2 Multi-processor scheduling a mem b b os 3 4 mem a os 2 1 scheduling

SoC-MOBINET courseware(c) Jan Madsen3 Scheduling of distributed real-time systems  Much more complicated  Has to handle a large number of characteristics, constraints and requirements

SoC-MOBINET courseware(c) Jan Madsen4 Characteristics to be handled  Processes:  preemptive and non-preemptive  periodic and aperidoic  processes with multiple levels of importance  group of processes with single deadline  Constraints and requirements:  end-to-end timing  precedence relations  communication  shared resources  placement  hard and soft deadlines  fault tolerance

SoC-MOBINET courseware(c) Jan Madsen5 Handling interfaces between  CPU scheduling and resource allocation  IO scheduling and CPU scheduling  CPU scheduling and communication scheduling  Local and distributed scheduling  Static scheduling of critical processes and dynamic scheduling of other processes Comprehensive results will not exist for many years!

SoC-MOBINET courseware(c) Jan Madsen6 Phases of real-time distributed systems scheduling  Allocation  assigning processes and resources to processors in the system  Scheduling  Ordering the execution of processes and communications to meet timing and resource constraints  Dispatching  Executing the processes in conformance with the scheduler’s decisions

SoC-MOBINET courseware(c) Jan Madsen7 Approaches to embedded system scheduling  deterministic (static) scheduling  Allocating and static scheduling all periodic processes off-line  Allocate time-slots for handling non-periodic processes  Distributed scheduling  Each CPU runs its own scheduler  Set of processes is distributed, statically allocated but dynamically activated

SoC-MOBINET courseware(c) Jan Madsen8 Static distributed scheduling  Typical approach –Construct a comprehensive graph containing all process instances that executes in an interval of length T (T is the least common multiple of all process periods) –Cluster communicating processes –Allocate clusters of processes to PEs and schedule processes and communications

SoC-MOBINET courseware(c) Jan Madsen9 ArchitectureSpecification CharacterizationScheduling A Simple Example t1 t3t2 PE1 PE2 bus t1 t2 t3 PE1PE2PE1 PE2

SoC-MOBINET courseware(c) Jan Madsen10 First solution t1 t3t2 PE1 PE2 bus ArchitectureSpecification CharacterizationScheduling t1 t2 t3 PE1PE2PE1 PE t1t2 t3 20

SoC-MOBINET courseware(c) Jan Madsen11 Introducing communication t1 t3t2 PE1 PE2 bus ArchitectureSpecification CharacterizationScheduling t1 t2 t3 PE1PE2PE1 PE t1 t2 t3 25 bus Can transfer 2 data units per time unit 5

SoC-MOBINET courseware(c) Jan Madsen12 Introducing interfaces t1 t3t2 PE1 PE2 bus ArchitectureSpecification CharacterizationScheduling t1 t2 t3 PE1PE2PE1 PE i1i2 t1t2 t3 27 bus i1 i2 Can buffer 10 data units per time unit

SoC-MOBINET courseware(c) Jan Madsen13 PE Memory? t1 Memory contributions: program data data to be transferred Memory utilization: time Memory size PE t1

SoC-MOBINET courseware(c) Jan Madsen14 Introducing memory t1 t3t2 PE1 PE2 bus ArchitectureSpecification CharacterizationScheduling t1 t2 t3 PE1PE2PE1 15 5,515 10,5 10 5,520 10,5 5 10, i1i2 t1t2 Peak memory = 77

SoC-MOBINET courseware(c) Jan Madsen15 Dynamic multi-processor scheduling  Primary objective is to ensure that all timing constraints are met  Serious difficulties in validating hard timing constraints  Assumptions:  Each processor has its own scheduler  The scheduler uses a uni-processor scheduling algorithm  Schedulers on different processors need not use the same algorithm

SoC-MOBINET courseware(c) Jan Madsen16 Multi-processor scheduling  Task assignment  Most real-time systems are static, tasks are partitioned and statically bound to processors  Inter-processor synchronization  Ensuring that precedence constraints of tasks on different processors are always satisfied

SoC-MOBINET courseware(c) Jan Madsen17 Task assignment  Often done off-line  NP-hard problem  Assignment based on:  Execution times  Resource requirements  Data dependencies  Timing constraints  Communication cost ... b os a

SoC-MOBINET courseware(c) Jan Madsen18 Task assignment - RMFF Assume zero communication overhead Scheduling based on fixed priorities A synchronization signal makes sure that is released as soon as has completed 32  Example: 1 a i k i k  Rate-Monotonic First-Fit algorithm: 1.Sort tasks in increasing order according to their period 2.Assign tasks one by one  is assigned to  is assigned to if the total utilization of and the tasks already assigned to < U RM

SoC-MOBINET courseware(c) Jan Madsen19 Example riri TiTi eiei b os 4 a 1 2 3

SoC-MOBINET courseware(c) Jan Madsen20 Dynamic scheduling b os 4 a a b RM

SoC-MOBINET courseware(c) Jan Madsen21 Changing synchronization protocol b os 4 a a b RMEDF

SoC-MOBINET courseware(c) Jan Madsen22 Multi-processing anomalies  Assume a set of tasks optimally scheduled on a multiprocessor system with:  fixed number of processors  fixed execution times (e i )  precedence constraints  Then  changing the priority list  increasing the number of processor  reducing execution times  weakening the precedence constraints  May increase the scheduling length!

SoC-MOBINET courseware(c) Jan Madsen23 Example of anomalies Reduce e 1 of task 1 a b a b Task 2 and 4 are sharing a resource, i.e. mutually exclusion

SoC-MOBINET courseware(c) Jan Madsen24 Consequences of anomalies  Tasks may complete before their WCETs  So most on-line scheduling algorithms are subject to experience anomalies  Simple but inefficient solution:  Have tasks completing early idle

SoC-MOBINET courseware(c) Jan Madsen25 Summary  Introduction to classical uni-processor multi-task scheduling  illustration of multi-processor scheduling  Not covered:  Priority inversion when having shared resources  Context switching and cache overhead  Overload conditions  Low power scheduling,  Increasing idle periods on processors  Voltage scaling  Communication